There are three major types of pulping methods known in the Pulp and Paper Industry. The first is chemical and the second is mechanical and the third is a combination of chemical and mechanical. Methods to enhance the effectiveness of all three types of pulping methods are always desirable.
In chemical pulps, sufficient lignin is dissolved to allow the fibers to separate with little, if any mechanical action. However, a portion of the lignin remains with the fiber and an attempt to remove this during digestion would result in excess degradation of the pulp. The degradation is a depolymerization of the cellulose and is measured by determining the viscosity of the cellulose dissolved in special solvents. For this reason from about 3 wt. % lignin to about 4 wt. % lignin is normally left in hardwood chemical pulps and from about 4 wt. % to about 10 wt. % lignin is normally left in softwood chemical pulps after the cook or digestion. The lignin is subsequently removed by bleaching in separate pulp mill operations if completely delignified and whitened pulps are to be produced.
The dominant chemical wood pulping process is the kraft (“kraft” means strength in German) or sulfate process. In the kraft process, the alkaline pulping liquor or digesting solution contains about a 3 to 1 ratio of sodium hydroxide and sodium sulfide. A stronger pulp is obtained when sodium sulfide is used in combination with sodium hydroxide. This is to be compared with pulp obtained when sodium hydroxide is used alone, as it was in the original soda process. In the kraft process the wood is delignified (pulped) with a solution of sodium hydroxide and the addition of sodium sulfide is beneficial for pulping as well. Key advantages of the kraft process is its great adaptability of pulping many different species of wood and yielding pulps that may be used for a variety of applications.
Another type of chemical pulping is the “sulfite process”. The sulfite process has several advantages over the kraft process. These advantages include improved yield (45-55%), lower cost cooking chemicals, higher brightness pulps and more easily bleached pulps. However, the sulfite method also has two distinct disadvantages: only a limited number of species can be pulped and the pulps produced are distinctly weaker than those made using the kraft or sulfate process.
In mechanical pulping, pulp is made predominantly using mechanical methods. The fundamental criteria used in assessing the quality of mechanical pulp is the amount of energy expended per unit of production. Because this energy is difficult to quantify, pulp freeness is most commonly used as a process control parameter. Generally, the more the energy expenditure the lower the freeness of the pulp.
The first step in the mechanical pulping process is the grinding or refining of wood.
The Stone Groundwood (SGW) process involves making pulp by pressing logs and chips against an abrasive rotating surface. Many years ago the grinding surface used was an actual stone. In current practice specifically designed “artificial pulp stones” are available for the grinding.
A Pressurized GroundWood (PGW) process is where the grinding operation is completely pressurized.
Another type of mechanical pulping is Refiner Mechanical Pulp (RMP) featuring atmospheric refining with no pretreatment of the wood chips.
Thermo Mechanical Pulping (TMP) is a mechanical pulping process that evolved from RMP and a high temperature process known as the Apslund process. Thermo Refiner Mechanical Pulping (TRMP) is a variation in Thermo Mechanical Pulping. In this case, the chips are preheated under pressure and refining is carried out at atmospheric pressure. TMP and TRMP pulps are stronger than either SGW or RMP pulps.
The third type of pulping process is a combination of chemical and mechanical pulping processes. Two types of combination processes are ChemiMechanical Pulping and SemiMechanical Pulping. There is little difference between ChemiMechanical Pulping (CMP) and SemiChemical Mechanical Pulping (SCMP). Both processes involve pretreatment of chips with chemicals, followed by mechanical refining. Four different chemical treatments are associated with these processes. These chemical treatments are: sodium hydroxide, sodium bisulfite, sodium sulfite, acid or its known salts sulfite treatment. These processes are generally used on hardwoods. Chemical treatment weakens the fiber structure allowing fibers to rupture similarly to softwood that is mechanically pulped.
ChemiThermoMechanical Pulping (CTMP) appears to be a full evolution of all Mechanical pulping methods. It includes chemical treatment elevated temperature steaming followed by mechanical refining. This process can produce fibrous raw materials that vary considerably in properties depending upon process conditions such as sodium sulfite concentration, pH, temperature, etc.
With all pulps, “pulp brightness” is a measurement of the ability of a sample to reflect monochromatic (457 nm) light as compared to a known standard, using magnesium oxide (MgO). Since cellulose and hemicellulose are white, they do not contribute to pulp color. It is generally agreed that the lignin left in the pulp after pulping is responsible for the color the pulp. This unbleached pulp has an appearance similar to brown grocery bags. The chromophores are believed to be quinone-like materials formed from the lignin's phenolic groups through an oxidative mechanism. Additionally, heavy metal ions, especially iron and copper, can form colored complexes with the phenolic groups.
There are generally two approaches to removing color. The first, typical of processing of mechanical pulps, uses a selective chemical to destroy the chromophores but not the lignin. The other approach, typical of processing chemical pulps, uses a bleaching system to remove the residual lignin. The bleaching of pulp is the standard method of removing color from pulp. It is current state of the art technology for all chemical and mechanical pulps to be bleached.
In chemical pulp, the bleaching of pulp and the subsequent delignification of pulp is usually performed in several chemical stages, with each stage being referred to by a letter designation. Note, that although all pulps are bleached, only chemical pulps are delignified using oxygen treatment.
The following table briefly describes the most common stages in a “typical” chemical bleaching process. Note that the stages captured in this table are not necessarily in the order that they are practiced. For example, oxygen delignification is typically never the last step in the process as oxygen delignification leaves the pulp yellowish in color. That is why oxygen delignification is followed by some level of bleaching.
StageDescriptionC—chlorinationReaction with Cl2 in an acid or its known saltsmediumE—ExtractionDissolution of chlorination reaction productsorwith sodium hydroxideEOAdding oxygen with the sodium hydroxide toorimprove delignification and lower the use ofchlorine and chlorine dioxideEOPAdding oxygen and peroxide with the sodiumhydroxide to improve delignification and lowerthe use of chlorine and chlorine dioxideH—HypochloriteReaction with sodium hypochlorite in alkalinemedium, used to bleach both chemical andmechanical pulpsY—HydrosulfiteReaction with sodium hydrosulfite in mildlyacetic--neutral conditions, used to bleachmechanical pulpsD—Chlorine DioxideReaction with ClO2 in an acid or its knownsalts mediumP—PeroxideReaction with peroxides in an alkaline mediumO—OxygenReaction with O2 at high pressure in an alkalinemedium. Usually used prior to chlorine as adelignification step.DC or CDMixture of chlorine and chlorine dioxide
Five or six stages are needed to produce a “full bleach” brightness level of 89 to 91% MgO. Most commonly these stages, in order are CEDED, CEHDED and OCEDED. A brightness of 65% MgO can be obtained with less stages, usually a CEH. Intermediate brightness levels can be reached using CED, CEHH, CEHD, or CEHP. Brightness enhancement during bleaching of pulp, as well as improving selective lignin removal during oxygen delignification of the chemical (kraft) pulp is important in the pulp and paper industry. Brightness enhancement is also useful in mechanical pulps.
It is to be understood that separate from the technical aspects of bleaching pulp there are environmental concerns that have dictated that chlorination has been almost entirely eliminated in favor of alternative treatments.
In current practice in pulp and paper mills, mechanical pulps are not oxygen delignified.
Currently, hydrogen peroxide is the dominant bleaching agent for mechanical pulps. Sodium hydrosulfite is also used for bleaching. It is known that hydrogen peroxide and hydrosulfite gradually decompose during the process due to unproductive side reactions catalyzed by transitional metal ions. Therefore, metal management through chelation is considered a key to increased brightness.
Several auxiliary chemicals are needed to provide an adequate performance. These auxiliary chemicals include sodium silicate for stability and chelation, sodium hydroxide for alkalinity, chelating agents such as ethylenediaminetetraacetic acid or its known salts (EDTA) and diethylenetriamine pentaacetic acid or its known salts (DTPA) for control of transition metals, and magnesium sulfate for cellulose stability. Each chemical added increases the cost of the bleaching method. High loads of bleaching chemicals can often cause downstream problems in papermaking.
Although the benefits of using a chelant are known in the pulp and paper industry, the known chelants used in hydrogen peroxide bleaching:                (1) are usually selective in regards of the target transition metal ions (e.g., removing manganese but not iron or vice versa);        (2) must be applied in substantial quantities to achieve a noticeable effect; and        (3) require washing out complexes formed during the treatment.        
Selectivity, as it applies in oxygen delignification, is defined as the ratio of the change in delignification (kappa number that characterizes lignin removal; the lower the better) divided by the change in viscosity (that characterizes carbohydrate depolymerization, the higher the better). Currently, the commonly used chemical for increasing the selectivity of oxygen delignification is magnesium sulfate. Magnesium sulfate does not influence the delignification, but provides a small measure of protection for the pulp viscosity. Conventional chelants such as DTPA and EDTA are also used for this purpose, however, none of them are reported to affect the kappa number. Therefore, currently, there are no known additives that can provide a noticeable improvement in lignin removal during oxygen delignification.
Japanese Patent Application No. 4-114853 discloses a method of pretreatment of wood pulp before bleaching which uses certain water-soluble polymers. The desired goal is to subject wood pulp to bleaching pretreatment through inexpensive pretreatment having little toxicity, thereby permitting a high degree of bleaching of wood pulp in the subsequent bleaching step. The recommended amount of polymer used in this Japanese Patent Application is from about 0.04 to 0.8 wt % per “exsiccated” pulp.
U.S. Pat. No. 6,702,921 issued on Mar. 9, 2004 and is entitled Method To Enhance Pulp Bleaching And Delignification. This patent describes and claims a method for making a Chemical or Mechanical pulp comprising the steps of digesting wood chips in the digester(Chemical) or grinding wood chips using mechanical grinding techniques (Mechanical) to create unbleached pulp and then bleaching the pulp and optionally subjecting the pulp to pressurized oxygen delignification(only with Chemical pulps); where the improvement comprises treating the pulp with from about 0.002 weight % to about 0.02 weight % of an organic sulfide chelating agent before or during bleaching, or before optional oxygen delignification of the pulp.
It would be desirable to identify additional or alternative compounds capable of brightness enhancement during bleaching of mechanical and chemical pulp.